However, there are a few possibilities, depending on how broadly you define "monster":
* Ogres: While not explicitly called "ogres" in Tolkien's works, the "goblin-men" of Moria, the "wild men" of the East, and even some "orcs" in their more monstrous forms, could be considered "ogre-like" in their physical descriptions and brutality.
* Olifants: These giant elephant-like creatures, while not inherently monstrous, are often associated with evil due to their use by Saruman and the forces of Mordor.
* Orcs: While a commonly known Tolkien creature, "orc" doesn't start with "O".
If you have more details about the monster you're thinking of (like its appearance, abilities, or what book it's from), I might be able to help further.